[ODE] three-body joints

[Alen Ladavac]

> The differential is a joint between the drive shaft, and the two axels
> that go to the wheels.
Yes, that's exactly what I ment. But if the drive shaft is not an ODE body,
I thought you might try to factor the whole thing into a motor joint. That
joint would just connect two wheels and current motor parameters would be
inputs for the joint. But then again, I wasn't thinking about this:
>  Actually, since some angular momentum is
> transferred to the chassis as well, I guess it has to be a 4-body joint.
Yes, the piece where momentum of wheel rotation is actually supported by the
chasis is important there, of course.

> It might in theory be possible to use only 2-body joints by modeling the
> gears in the differential as trimeshes,
Hm... why trimeshes? If you are into modeling contacts between gears, you
could just hard-code them as permament contacts. It does sound like
overdoing, but since you are already modeling the flywheel and clutch... :)

Just my 2 cents,
Alen

> but that's going to be a whole
> can of worms.  I can't see that being at all practical.  Anyway, no
> rush, my current system is acceptable for now :)  ODE still rocks for
> the suspension model :)
>
> - Graham Fyffe
> president, Happy Digital
>
>
>
> > If it's only about the differential, you should probably implement it as
a
> > two body joint. As I understand it, the third body here (motor) is not
> > really a body in a usual simulation. I mean, you don't actually model
the
> > engine, transmission, etc. directly with bodies and joints, do you? ;)
> Or do
> > you need differential joint for something other than powering a car?
> >
> > Alen
> >
> > ----- Original Message -----
> > From: "Gary R. Van Sickle" <g.r.vansickle at worldnet.att.net>
> > To: <ode at q12.org>
> > Sent: Friday, September 10, 2004 01:49
> > Subject: RE: [ODE] three-body joints
> >
> >
> > > > Hi everybody.  I'm wondering, what are the design issues that
> > > > caused Russ to implement only two-body joints?  It seems to
> > > > me that, from the point of view of the "big matrix", it's
> > > > just as easy to implement n-body joints.  For example, the
> > > > differentials used in cars can only be implemented as 3-body
> > > > joints.  No combination of 2-body joints will do.
> > >
> > > Really?  I'm having a hard time coming up with a reason why; can you
> > explain
> > > further (or point me to a links)?
> > >
> > > >  The equations for a differential are in the form e.g. c1 wA + c2 wB
+
> > > > c3 wC = 0, for bodies A, B, C.  What would it take to add
> > > > n-body joint support?
> > >
> > > Somebody to code it up and maintain it.  I can't speak for Russ, but
I'm
> > > sure if it fit in well and was truly useful it would have a pretty
fair
> > > chance of at least getting into "contrib".
> > >
> > > --
> > > Gary R. Van Sickle
>
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